Cellular or wireless phones of the “clamshell” and “flip” type have become increasingly popular in recent years. Such designs can present unique challenges in the area of antenna design. Depending on the nature of the “flip” cover and the particular antenna configuration being used, the position of the flip cover can have a significant impact on the performance and matching characteristics of an antenna system. For example, the electrical ground plane presented by the chassis of the mobile terminal can nearly double when a flip cover is moved from the closed position to an open position. Electrical connections through the hinge may increase coupling to the antenna when the flip cover is open. An antenna system and its interface can be optimized for one configuration. However, changing to the other configuration can often be seen as a degradation of the voltage standing wave ration (VSWR) of the antenna. Such degradation results in poor performance and may in fact be unacceptable for some multiband antenna systems.
In some cases, designers make an assumption that one position is the most important, for example the open position in which most people operate a phone for voice communication. Thus, an antenna system is matched to perform best with a clamshell or flip cover phone in that position. Unfortunately, the same matching circuit and/or feed system can result in significantly degraded performance in the other operating position. FIG. 1 illustrates this situation. FIG. 1 is divided into FIGS. 1A and 1B for clarity. FIG. 1 shows a VSWR curve, 100, of an example mobile terminal with a two-band antenna. The VSWR is plotted on vertical axis 102, and frequency is plotted on horizontal axis 104. The mobile terminal in question operates on frequency band A and frequency band B, both marked on horizontal axis 104. Performance considerations in this example dictate a VSWR of less than 2.5 to 1 be maintained for the frequency ranges of interest. As can be readily seen, VSWR curve 100 has minimum VSWR points 106 and 108 occurring near the center of frequency bands A and B. As can also be seen, the VSWR remains substantially below the 2.5 to 1 goal over the entire operating range of frequency bands A and B, as indicated by portions of the VSWR curve being below line 110 in the graph of FIG. 1A. FIG. 1A illustrates VSWR for a mobile terminal with its flip cover in one position.
FIG. 1B illustrates VSWR curve 120 plotted on the same axis as illustrated in FIG. 1A, as evidenced by like reference numbers. FIG. 1B illustrates a VSWR graph for the same mobile terminal with its flip cover in the other position. The minimum VSWR achieved in band A is approximately 6.1 to 1, as shown at point 122. The minimum VSWR achieved in frequency band B is just over 2.5:1, as illustrated by point 124. In this example, antenna performance degrades significantly when the flip cover of the mobile terminal is moved to the other position.